1. Field of the Invention
The present invention relates to a method and device capable of electrostatically investigating a surface and/or sub-surface structure. In particular but not exclusively, the method and device in accordance with the present invention are capable of sensing variations in the composition and density of material structures, and the presence of an internal mass in a building structure.
2. Brief Description the Prior Art:
A known method, capable of carrying out scientific measurements of dielectric constants in wood and materials containing wood, uses a pair of electrically conductive plates of the same dimensions applied on opposite sides of the wooden sample to fill all the space between the plates with this sample. An alternating current is then applied to the capacitor formed by the two plates and the sample. Theoretically, the capacitor may be applied to several points of a piece of wood to detect changes in the dielectric constant thereof. However, this procedure presents numerous disadvantages:
the plates along with the supply leads have to be applied to opposite sides of the wooden sample, which is often impracticable; PA1 filling the space between the two plates with the sample is impossible when an internal member of an already erected building structure should be inspected; PA1 electrocution hazards exist as conductive material is generally present in the structures under investigation; and PA1 the measurements must be compared to known values, which requires calibration. PA1 the stud sensor requires calibration to be operational; PA1 if the initial calibration is made over a point to be detected, that is a point of high density corresponding to the position of a stud, the stud sensor will not detect the studs; it will not detect the points of high density as long as the calibration is made over a point of low density through trial and error; PA1 after successful calibration, the stud sensor will indicate points on the wall where the density is higher than that of the point of the last calibration; PA1 frequent calibration may be necessary; PA1 the stud sensor will not produce meaningful readings or may give misleading indications in the presence of thicker surface material, as is often the case in wood lath and plaster walls and in gypsum board walls and ceilings where thick layers of plaster are present to level off joints; PA1 it cannot operate meaningfully on wooden board or plank finish, because of the thickness of such a finish and the generally significant variations in density from one board or plank to the other; PA1 it will not operate either in the presence of electrically conductive fasteners used in walls and ceilings, and more generally in the case of wood board finish floors; PA1 within its range of sensitivity, it will signal the presence of electrically conductive materials, without depth discrimination; PA1 it presents a low resolution in the detection of conductors. PA1 the sensing device comprises a plurality of electrically conductive second sensor plates electrically connected together; PA1 the first plate and the plurality of second plates are mounted on a common plate support; PA1 the first plate is a central electrically conductive sensor plate presenting two opposite edges, and the plurality of second plates comprises two side electrically conductive sensor plates mounted adjacent the two opposite edges of the central plate, respectively; PA1 the first plate is a central electrically conductive sensor plate formed with three edges, and the plurality of second plates comprises three side electrically conductive sensor plates mounted adjacent the three edges of the central plate, respectively; PA1 the first plate comprises a central electrically conductive sensor plate formed with four edges, and the plurality of second plates comprises four side electrically conductive sensor plates mounted adjacent the four edges of the central plate, respectively; PA1 (a) the central and side plates are coplanar, or (b) the central plate is planar and each side plate defines an acute angle with the plane of the central plate to orient one face of the side plate toward a zone located beneath the central plate. PA1 (a) the central plate is square, (b) the edges of the central plate are straight and have a given length, (c) the side plates are rectangular and elongated, and have a length equal to the length of the edges of the central plate, and (d) each side plate is parallel to the corresponding edge of the central plate; PA1 when the sensing device comprises four side plates, a switching means may be provided to disconnect at least one of the four side plates from the three other ones.
An adaptation of the above method is the application of a pair of capacitive plates on the same face of a tested material. As the intensity of the electric field is inversely proportional to the square of the distance between electrically conductive plates charged with opposite polarities, the sensitivity of such capacitive sensors and the applications thereof are severely restricted.
An "electronic wall stud sensor" is described in U.S. Pat. No. 4,099,118 granted to Franklin et al. on Jul. 4, 1978. This sensor includes two sets of electrically conductive plates applied on the surface of a wall to be investigated. The plates of the two sets are charged at different rates and the sensor measures the difference in the time for the plate sets to reach a given voltage threshold, this time difference varying with the density of the sub-surface density. Although the stud sensor of Franklin et al. may be effective in some applications, it however presents the following drawbacks:
U.S. Pat. No. 4,992,741 granted to Douglas et al. on Feb. 12, 1991, proposes a device for detecting objects behind a wall. This detecting device comprises a plurality of capacitor plates at spaced intervals and a circuit for sensing alterations in the dielectric constant of a region of the surface being inspected close to each capacitor plate. Display elements are respectively associated to the capacitor plates to display the signals detected through these plates and image the object being detected. Although this detecting device may be efficient in some applications, its sensitivity is limited since, as discussed hereinabove, the intensity of the electric field is inversely proportional to the square of the distance between the plates charged with opposite polarities. The device of Douglas et al. is also susceptible of performing erroneous readings due to the user's position with respect to the detecting device. Finally, it is complex and expensive.